In electrically driven linear actuator used in many types of driving mechanisms of a vehicle, such as an automobile and the like, a trapezoidal screw thread or a gear mechanism, such as a rack and pinion, are used as the mechanism to convert the rotary motion of the electric motor to the linear motion in an axial direction. These converting mechanisms usually have sliding contact portions and thus a defect of power loss, which needs power up of the electric motor and thus is obliged to increase the power consumption. Accordingly, ball screw mechanisms have been increasingly used as a more efficient actuator.
The ball screw is a mechanical element to convert a rotational movement to an axial translation motion, a linear motion. The ball screw includes a ball screw shaft with a helical ball screw groove formed on its outer circumferential surface. A ball screw nut has a helical ball screw groove formed on its inner circumferential surface. A plurality of balls are contained in a ball raceway formed between the helical ball screw grooves of the ball screw shaft and nut.
The reciprocal stroke of the ball screw actuator is finite and is several millimeters. In order to prevent overrunning of the linear motion of the ball screw shaft or the ball screw nut, it is necessary to provide mechanical stoppers at both ends of the stroke. Although it is possible to electrically control the stroke of the actuator using a limit sensor, the mechanical stoppers are required to prevent an accidental overrunning of the actuator.
The ball screw mechanism acts as a boosting mechanism to convert a rotational force to a thrust force. Accordingly, a large thrust force is generated between an abutting member and the ball screw shaft or the ball screw nut when either one of them strikes against the abutting member at the end of the stroke. Since the ball screw shaft or the ball screw nut strikes against the abutting member during its rotation, it bites the abutting member due to its wedging action against the abutting member. Once it has been rigidly bitten, it is difficult to release from the bitten condition if trying to drive the electric motor in a reverse direction. This is due to the usual design of the electric motor which leaves less torque margin.
One method of releasing the bitten rigid engagement is proposed in Japanese Laid-open Patent Publication No. 186065/1995. Here a screw arranged within a driving screw shaft can be rotated to release the bitten engagement. Although detailed description of this method is not therein, it may be easily supposed that the bitten engagement is released by adding a large torque on the screw using a dedicated tool having a large arm ratio.
However there are several problems in the prior art. When the ball screw shaft or the ball screw nut strikes against the abutting member at the end of the stroke, it bites the abutting member due to its wedging action against the abutting member. According to the Japanese Laid-open Patent Publication No. 186065/1995, there are also problems that the strength of the ball screw shaft is reduced due to forming of screw threads within the shaft. Also, use of a dedicated tool is required, in order to release the bitten condition.